Rack or sub-rack type casings for enclosing plug-in modules (module substrates) have been well known, as casings for electronic apparatuses that are compliant with standards, such as the Compact Peripheral Component Interconnect (Compact PCI), the Versa Module Eurocard (VME), and the Advanced Telecom Computing Architecture (ATCA).
Inside such a casing, multiple slots are provided in a row for attaching plug-in modules. Each slot is provided such that a rail for supporting an edge of a plug-in module extends perpendicular to the backplane, and plug-in module is attached or detached to the backplane by sliding the plug-in module on the rail.
A wide variety of types of plug-in modules are used. For example, a central processing unit (CPU) module, a storage module, a power supply module, and the like are used for a server apparatus or a disk array. For a network communication apparatus, interface modules and the like, which are compliant with a wide variety of communication standards, are used (see Patent Reference 1, for example).
A cooling fan for air-cooling plug-in modules and airflow openings formed in positions corresponding to the slots are provided in a casing. The cooling fan inhales the air from airflow openings defined in the bottom or sides of the casing, for example, and exhausts the air from the rear of the casing, thereby establishing the air flow that cools the plug-in modules. Another structure is also known wherein a shutter plate is provided for each airflow opening, and the airflow opening is opened only when a plug-in module is inserted in the slot. In this structure, the air-cooling efficiency can be increased while blocking undesired air flow since airflow openings corresponding to unused slots are closed by shutter plates and shutters are opened only for used slots (see Patent References 2 and 3, for example).    Patent Reference 1: Japanese Laid-open Patent Publication No. 2004-252758    Patent Reference 2: Japanese Laid-Open Patent Publication No. 05-299861    Patent Reference 3: Japanese Laid-Open Patent Publication No. 06-164179
Different plug-in modules generate different heat amounts. An ideal amount of air flow for some modules may be smaller than for other modules. However, since conventional air-cooling structures open and close airflow openings depending on whether the slots are used or not, without taking the amount of exhaust heat of each plug-in module into account, the air flow inside the casing as a whole is not well-balanced. As a result, the rotational speed of the cooling fan may be increased, in order to create sufficient air flow near a plug-in module generating the greatest amount of heat.
Alternatively, it is envisioned to adjust the air flow inside the casing by varying the rotational speeds of the individual cooling fans. However, such a technique uses complex structures and increases the cost. Sensors and control devices (such as a CPU and a memory) that controls the rotational speed of each cooling fan are expensive, since such sensors and control devices check the type and the amount of exhaust heat of each plug-in module. Further, the reliability remains low since other cooling fans are usually disposed to cool down the control devices, while the control devices are disposed for improving the cooling performance of the casing.